Oxidation reactions are reactions that are most frequently utilized in the chemical industry from bulk chemicals to fine chemicals, and a number of studies are being conducted on these reactions.
With respect to oxidizing agents used in the oxidation reactions, their raw materials range from the most inexpensive oxygen to nitric acid, hydrogen peroxide, and metal oxides in the order of increasing cost, and these oxidizing agents are used in the production of a variety of useful compounds.
For example, adipic acid, which is a raw material of Nylon 66, is currently produced by a method in which cyclohexanol alone or a mixture of cyclohexanol and cyclohexanone (KA oil) is oxidized with nitric acid. In such nitric acid oxidation, nitrous oxide (N2O) and NOx, which are said to have global-warming potentials at least 300 times higher than that of carbon dioxide, are formed as by-products. Thus, expensive exhaust-gas processing facilities are needed to process these by-products.
In view of this, as a result of focusing on the fact that KA oil is obtained by way of catalytic oxygen oxidation of cyclohexane and studying this catalytic reaction, a method whereby adipic acid is directly obtained from cyclohexane has been developed. Although the use of the most inexpensive molecular oxygen or air is industrially ideal, it has not yet been used for industrial purposes because of its insufficient conversion and selectivity.
Direct oxidation of cyclohexene to adipic acid using hydrogen peroxide solution is also being studied. Although the yield is good, and hydrogen peroxide does not give off any harmful by-products such as in nitric acid oxidation, hydrogen peroxide is far more expensive than nitric acid. Therefore, this method has not yet been used for industrial purposes.
On the other hand, it has been reported that adipic acid can be obtained by ozonizing cyclohexene, followed by treatment with hydrogen peroxide (Non-Patent Literature 1). Ozone can be obtained by, for example, subjecting oxygen to silent discharge, and the ozonization reaction can be conducted without a catalyst because ozone is highly reactive. Ozonolysis is considered to be an excellent oxidation reaction for industrial purposes, such as for the production of adipic acid by ozonolysis of cyclohexene, in view of resources and waste, because a carboxylic acid can be derived by oxygen oxidation or the like, instead of treating an ozonide with hydrogen peroxide.
However, it has been reported that, in the ozonolysis of an organic compound, “the risk of an explosion must always be kept in mind because an organic peroxide is produced” (Non-Patent Literature 2); therefore, the reaction has almost never been conducted on an industrial scale. Even in the reaction on a small scale in a laboratory, isolation and purification of an ozonide produced by ozonization has been considered to be dangerous.
Thus, many studies have been conducted in order to safely handle ozonolysis. Examples of proposed methods include a method in which, during ozonization, a fatty acid is added to a raw material olefin to control the rate of production of an undesired unstable peroxide, and the proportion of the produced peroxide is monitored using NMR, thereby ensuring the safety of oxidation decomposition by oxygen subsequent to the ozonization (Patent Literature 1); a method in which, after the ozonization reaction or reduction treatment of an ozonide using platinum/hydrogen, purification is conducted by steam distillation (Patent Literature 2); and a method in which ozone is contacted in a micro-reactor, thereby efficiently removing the reaction heat due to ozonization (Patent Literature 3).
The ozone gas that has been used in many of these ozonization methods is a mixed gas containing about 3% of ozone and the remaining 97% of oxygen and obtained by an ozone generator using oxygen as a raw material. In the ozonization reaction of double bonds, it is believed that ozone undergoes an addition reaction with double bonds to produce a molozonide, which subsequently undergoes a rearrangement that involves cleavage of carbon-carbon bonds, thus producing a so-called ozonide (Non-Patent Literature 3). It is believed that, during this rearrangement of the molozonide, oxygen present in excess of ozone forms radicals to produce an undesired unstable peroxide, thus making the reaction dangerous and complicated. Stille et al., in practice, conducted a reaction using ozone having a relatively low oxygen content by concentrating ozone by passing a mixed gas of ozone and oxygen, which exited from an ozonizer, through a silica gel to selectively adsorb the ozone, and subsequently desorbing the ozone with nitrogen gas. Consequently, they dramatically reduced side reactions (Non-Patent Literature 4). Ozonolysis in which the influence of oxygen has been reduced by generating ozone by corona discharge under a carbon dioxide stream without using oxygen has also been proposed (Patent Literature 4).